(1.) Field of the Invention
The present invention relates to an optical recording material for use in a frequency selective optical data recording and storage system in which multiplex recording on the same site of one material can be performed with lights differing in wavelength by utilizing the technique of photochemical hole burning.
(2.) Description of the Related Art
The photochemical hole burning (hereinafter referred to as "PHB") is a phenomenon in which, when a material causing a photochemical reaction at an ultra-low temperature such as the temperature of liquid helium is irradiated with a light having good monochromaticity, only the molecule absorbing this light is selectively excited to cause a photochemical change. Since sharp holes (dents) are formed in the light absorption spectrum of the material by this photochemical change, the formation of an optical memory becomes possible according to the presence or absence of the holes. Moreover, if recording is carried out in succession by using irradiation lights differing in wavelength, wavelength-multiplexed recording can be performed on the same site of one material. If this wavelength-multiplexed recording method is adopted, a possibility exists that the recording density will be improved to a level of about 1,000 times higher than the recording density attainable in a conventional optical digital recording medium such as a compact disk or a laser disk.
An optical recording material utilizing this PHB phenomenon comprises guest molecules, which are photoreactive compounds, and a host for dispersing these guest molecules. To increase the wavelength multiplicity in an optical recording, it is sufficient if the dispersion state of the guest is varied, and the use of an amorphous substance as the host is preferred for this purpose. Therefore, a polymer or silica glass has been used as the host.
As typical instances of the conventional PHB material, there are known a material comprising free-base porphine represented by the formula shown in FIG. 1 as the guest and an aliphatic hydrocarbon as the host; a material comprising tetraphenylporphine represented by the formula shown in FIG. 2 as the guest and a polymer such as polymethyl methacrylate as the host [Japanese Journal of Optics, 14, (4), 263-269]; a material comprising Cresyl Violet represented by the formula shown in FIG. 3 as the guest and polyvinyl alcohol as the host; a material comprising quinizarin represented by the formula shown in FIG. 4 as the guest and silica glass as the host [J. Appl. Phys., 58, (9), 3559-3565]; and a material comprising phthalocyanine represented by the formula shown in FIG. 5 as the guest and an aliphatic hydrocarbon as the host.
With respect to the temperature characteristics of PHB materials, Thijssen et al reported a formation of holes at temperatures lower than 30K [Chem. Phys. Lett., 92, (2), 7-12], and Tani et al reported a reservation of holes at temperatures lower than 60K [J. Appl. Phys., 58, (9), 3559-3565].
In the conventional PHB materials, the half width of the formed holes increases as the temperature rises and the wavelength multiplicity of a recording is drastically decreased. To put wavelength-multiplexed recording to practical use, for the reasons set forth below, it is necessary to develop a PHB material in which this defect is alleviated as much as possible.
In the first place, recording is made in the PHB material and is then read out, and the temperature of the material must be maintained at a low level during this period by using a coolant such as liquid helium. But, a temperature deviation inevitably occurs in the cooling apparatus, and the PHB material to be used must stably retain the recording state even if the material undergoes this temperature deviation. In the second place, the PHB material is irradiated with light for writing information, and at the time of irradiation, the temperature of the material is elevated by the absorption of light. This elevation of the temperature is conspicuous when the writing of information is carried out at a high speed by increasing the irradiation intensity. Therefore, the PHB material to be used must have properties such that recording can be stably performed even if the material is exposed to this elevated temperature.
Moreover, in the case of the conventional PHB materials, it is impossible to form holes at the temperature of liquid nitrogen. But, there is a demand for developing a PHB material in which holes can be formed at the liquid nitrogen temperature, since this is practically advantageous because the cooling cost is drastically reduced.